The invention relates to a thermoplastic vulcanizate.
Thermoplastic vulcanizates are blends that are comprised of a thermoplastic and a cross-linked elastomer. The process of cross-linking of the elastomer is carried out by dynamic vulcanization. The term xe2x80x9cdynamic vulcanizationxe2x80x9d is understood to relate to a process in which the thermoplastic material, the rubber and the cross-linking system are masticated while the rubber is being cross-linked. Examples of dynamic vulcanization are described in the patents U.S. Pat. No. 4,130,535 and U.S. Pat. No. 4,311,628. Laid-open patent specification DE-A 26 32 654 describes a blend that is comprised of a thermoplastic polyolefin, an EPDM rubber and any one of the cross-linking systems known from U.S. Pat. No. 3,806,558. The rubber is vulcanized to such a degree that that it no longer contains more than about three percent of rubber that can be extracted in cyclohexane at 23xc2x0 C. GB-A 2 007 683 describes a thermoplastic elastomer containing a thermoplastic, crystalline polyolefin resin and vulcanized EPDM. The rubber is cross-linked in a phenolic resin. The degree of cross-linking so achieved is higher than 97%. Furthermore, WO-A 98/58020 describes a TPE blend based on a thermoplastic olefin, an EPDM rubber and an ethylene-octene copolymer (EOC). The thermoplastic vulcanizates introduced in said published patent document are produced without plasticizing oils. Moreover, a novel type of cross-linking system is employed. The cross-linking system is comprised of a phenolic resin and an oxide based on Mg, Pb, or Zn. Said system leads in said process to partial cross-linking of the EOC. The drawback with the described TPE is a high degree of swelling of the oil, such swelling being conditioned by the formula. Furthermore, the flowability is deteriorated due to partial cross-linking of the EOC. Moreover, increased raw material costs have to be expected on account of the high polymer content.
It is pointed out in EP-B-0 107 635 that the mixing methods employed until that time for producing the dynamically vulcanized thermoplastic elastomers are not suitable for producing mixtures with good extrusion properties. Described is a single-stage process in which a twin-screw extruder with screws revolving in the same sense of rotation is employed, which, at high shearing rates of  greater than 2000 sxe2x88x921 and a dwelling time of  less than 2 minutes permits the production of soft thermoplastic elastomers with good extrusion properties.
The development and production of soft thermoplastic vulcanizates that can be produced at favorable cost is particularly important for the further advance of TPE materials in the field of application of the classical elastomers. Soft thermoplastic vulcanizate are understood in the following to be materials with a hardness of less than 70 Shore A (said value relates to measurements on extruded flat sections). Since the absorption of oil of the rubber phase is limited, minimum hardness values of about 50 Shore A can be obtained in connection with thermoplastic vulcanizates based on EPDM/PP by adding plasticizer oils (EP-A 0 757 077). Even in said hardness a range, the increasing amount of the oil component already leads to a decrease in the mechanical properties as well as to increased fogging values. Also, it may be possible that the oil sweats out on the surface of the molded component. The values specified herein in the following are given only by way of example. Based on 100 parts rubber, a thermoplastic vulcanizate with a hardness of 50 Shore A contains about 150 to 200 parts of oil. In order to keep the technical processing expenditure within economically justifiable limits when the plasticizer oil is mixed in, EPDM rubbers extended with oil are preferably used. Again, however, a drawback is also in this connection the fact that the EPDM-types of rubber extended with oil are expensive in comparison to the non-diluted types of EPDM. Furthermore, types of EPDM extended with oil are available in the market, which are extended with 75 or 100 parts and contain plasticizer oils that comprise aromatics, so that discoloration occurs during in the course of wheathering. Therefore, the production of bright and soft thermoplastic vulcanizates poses special problems. With the dynamic vulcanizates available in the market on the basis of EPDM/PP, discoloration can be restricted only by means of suitably high expenditure on the raw material side, but is not entirely avoidable.
The addition of styrene block copolymers, which leads to a reduction in the hardness as well, is expensive and, furthermore, reduces the weather stability.
The patent literature cited in the following introduces formulas and process variations that permit the production of thermoplastic vulcanizates. A very soft thermoplastic vulcanizate ( less than 60 Shore A, preferably  less than 45 Shore A) is described in EP-A 0 757 077. Said vulcanizate consists of two vulcanized rubbers EPDM and BR, or SBR or CR, and a larger amount of process oil. The vulvanized rubbers are present in the thermoplastic matrix in the form of a finely distributed dispersed phase. A soft thermoplastic vulcanizate with a partially cross-linked rubber phase is introduced in WO-A 97/39059, which preferably consists of a thermoplastic polyethylene (homo- or copolymer), an amorphous polypropylene, and an EPDM- or BR-rubber. The amorphous polypropylene is. preferably added only after the rubber has been dynamically vulcanized.
Finally, it is pointed to EP A 0 092 318 which shows a thermoplastic blend of polyolefin and an elastomeric softener. The main objective is to be able to produce polypropylene foils of high value on polyethylene-working machines.
Now, as the thermoplastic vulcanizates known until now are afflicted with the drawbacks of higher technical processing expenditure and/or connected with high costs, the invention is based on the problem of developing a more favorably priced, soft thermoplastic vulcanizate that exhibits an only minor tendency to discoloration in the course of weathering.
The problem is solved by a thermoplastic vulcanizate comprised of a composition disclosed herein wherein useful material parameters are also specified.
Furthermore, the problem of the invention consists in producing the thermoplastic vulcanizate as defined by the invention without increased expenditure in terms of process technology.
In conjunction, usefull process steps are disclosed.
The thermoplastic vulcanizate and the method for producing it as defined by the invention are now described in the following in greater detail.
According to claim 1, the thermoplastic vulcanizate is comprised of four components (A, B, C, D), notably
a thermoplastic (A);
a substantially non-cross-linked polyethylene (B);
an at least partially vulcanized rubber (C); and
a plasticizer (D); as well as the standard ingredients (E) of the blend, whereby the individual groups of materials are now introduced in the following in greater detail.